Permissions.

example.rb

# Example:

class Product < Struct.new(:user_id); end
class Account < Struct.new(:user_id, :members); end
class User    < Struct.new(:id); end

Permissions.for(Account) do
  def can_read?(user)
    members.include?(user)
  end
end

Permissions.for(Product, Account) do
  def can_manage?(user)
    user_id == user.id
  end
end

Permissions.for(User) do
  def can_manage?(target)
    target.user_id == id
  end

  def can_read?(target)
    true
  end
end

################################################################################

include Permissions::Helper

def current_user
  $user ||= User.new 1234
end

prod = Product.new current_user.id
acc = Account.new 7890, [1,2,3]

puts can?(:read, acc)
puts can?(:manage, acc)
puts can?(:something, prod)
puts can?(:manage, prod)
puts can?(:read, prod)

permissions.rb

module Permissions
  def self.for(*where, &block)
    where.each { |klass| klass.class_eval(&block) }
  end

  module Helper
    def can?(action, target, who=current_user)
      perm = "can_#{action}?"
      who.respond_to?(perm) && who.send(perm, target)
    end
  end
end